1. Technological Field
The present invention relates generally to the field of fiber optic transceivers and particularly to an optical transceiver module including a controller integrated circuit for serially communicating transceiver diagnostic information to a host via at least two pins that extend to or from the bottom of the optical transceiver module's housing.
2. Description of Related Art
The two most basic electronic circuits within a fiber optic transceiver are the laser driver circuit, which accepts high speed digital data and electrically drives an LED or laser diode to create equivalent optical pulses, and the receiver circuit which takes relatively small signals from an optical detector and amplifies and limits them to create a uniform amplitude digital electronic output. In addition to, and sometimes in conjunction with these basic functions, there are a number of other tasks that must be handled by the transceiver circuitry as well as a number of tasks that may optionally be handled by the transceiver circuit to improve its functionality. These tasks include, but are not necessarily limited to, the following:                Setup functions. These generally relate to the required adjustments made on a part-to-part basis in the factory to allow for variations in component characteristics such as laser diode threshold current.        Identification. This refers to general purpose memory, typically EEPROM (electrically erasable and programmable read only memory) or other nonvolatile memory. The memory is preferably accessible using a serial communication bus in accordance with an industry standard. The memory is used to store various information identifying the transceiver type, capability, serial number, and compatibility with various standards. While not standard, it would be desirable to further store in this memory additional information, such as sub-component revisions and factory test data.        Eye safety and general fault detection. These functions are used to identify abnormal and potentially unsafe operating parameters and to report these to the user and/or perform laser shutdown, as appropriate.        
In addition, it would be desirable in many transceivers for the control circuitry to perform some or all of the following additional functions:                Temperature compensation functions. For example, compensating for known temperature variations in key laser characteristics such as slope efficiency.        Monitoring functions. Monitoring various parameters related to the transceiver operating characteristics and environment. Examples of parameters that it would be desirable to monitor include laser bias current, laser output power, received power level, supply voltage and temperature. Ideally, these parameters should be monitored and reported to, or made available to, a host device and thus to the user of the transceiver.        Power on time. It would be desirable for the transceiver's control circuitry to keep track of the total number of hours the transceiver has been in the power on state, and to report or make this time value available to a host device.        Margining. “Margining” is a mechanism that allows the end user to test the transceiver's performance at a known deviation from ideal operating conditions, generally by scaling the control signals used to drive the transceiver's active components.        Other digital signals. It would be desirable to enable a host device to be able to configure the transceiver so as to make it compatible with various requirements for the polarity and output types of digital inputs and outputs. For instance, digital inputs are used for transmitter disable and rate selection functions while digital outputs are used to indicate transmitter fault and loss of signal conditions.        
FIG. 1 shows a schematic representation of the essential features of a typical prior-art fiber optic transceiver. The main circuit 1 contains at a minimum transmit and receiver circuit paths and power supply voltage 19 and ground connections 18. The receiver circuit typically consists of a Receiver Optical Subassembly (ROSA) 2 which contains a mechanical fiber receptacle as well as a photodiode and pre-amplifier (preamp) circuit. The ROSA is in turn connected to a post-amplifier (postamp) integrated circuit 4, the function of which is to generate a fixed output swing digital signal which is connected to outside circuitry via the RX+ and RX− pins 17. The postamp circuit also often provides a digital output signal known as Signal Detect or Loss of Signal indicating the presence or absence of suitably strong optical input. The Signal Detect output is provided as an output on pin 18. The transmit circuit will typically consist of a Transmitter Optical Subassembly (TOSA), 3 and a laser driver integrated circuit 5. The TOSA contains a mechanical fiber receptacle as well as a laser diode or LED. The laser driver circuit will typically provide AC drive and DC bias current to the laser. The signal inputs for the AC driver are obtained from the TX+ and TX− pins 12. Typically, the laser driver circuitry will require individual factory setup of certain parameters such as the bias current (or output power) level and AC modulation drive to the laser. Typically this is accomplished by adjusting variable resistors or placing factory selected resistors 7, 9 (i.e., having factory selected resistance values). Additionally, temperature compensation of the bias current and modulation is often required. This function can be integrated in the laser driver integrated circuit or accomplished through the use of external temperature sensitive elements such as thermistors 6, 8.
In addition to the most basic functions described above, some transceiver platform standards involve additional functionality. Examples of this are the external TX disable 13 and TX fault 14 pins described in the GBIC standard. In the GBIC standard, the external TX disable pin allows the transmitter to be shut off by the host device, while the TX fault pin is an indicator to the host device of some fault condition existing in the laser or associated laser driver circuit. In addition to this basic description, the GBIC standard includes a series of timing diagrams describing how these controls function and interact with each other to implement reset operations and other actions. Some of this functionality is aimed at preventing non-eyesafe emission levels when a fault conditions exists in the laser circuit. These functions may be integrated into the laser driver circuit itself or in an optional additional integrated circuit 11. Finally, the GBIC standard also requires the EEPROM 10 to store standardized serial ID information that can be read out via a serial interface (defined as using the serial interface of the ATMEL AT24C01A family of EEPROM products) consisting of a clock 15 and data 16 line.
Similar principles clearly apply to fiber optic transmitters or receivers that only implement half of the full transceiver functions.
The above described transceiver platform standards also set out packaging and size limitations of the optical transceiver module. Therefore, despite including the above described additional functionality, current optical transceivers must still conform to the packaging and size limitations laid out in the transceiver platform standards. For example, presently most small optical transceivers are either Small Form Factor (SFF) or Small Form Factor Pluggable (SFP) optical transceivers. SFF transceivers are smaller than the standard transceivers, such as LC, MT-RJ and MU, and generally have an array of pins that are soldered directly to a printed circuit board. SFP transceivers, on the other hand, can be plugged and unplugged from a host and are not directly soldered to the printed circuit board.
These standardized sizes of optical transceiver modules provide for interchangeability of the optical transceiver modules within larger electronic components. However, as additional functionality is added beyond that required by the transceiver platform standards, the circuitry required for such additional functionality must nevertheless be positioned within the same standardized package. This restricts the amount of additional functionality that can be added to standardized optical transceivers.
In addition, the input and output (I/O) pins or connectors extending from such standardized optical transceivers are also governed by the transceiver platform standards. For example, conventional SFF transceivers have two basic I/O pin configurations, namely a 2×5 pin configuration and a 2×10 pin configuration, where 2×5 indicates two rows of five pins and 2×10 indicates two rows of ten pins. The functionality of each of these pins is also generally dictated by the transceiver platform standards. This restricts access to the additional functionality from an external host, as the standardized number of pins only provide for the I/O requirements of the standard optical transceiver module. In other words, no current mechanism exists for accessing such additional functionality, while retaining the existing footprint and pin locations set by existing transceiver platform standards.
The transceiver platform standards thus operate to restrict, if not prevent, access to additional functionality from an external host, since the standardized number of pins only provide for the I/O requirements of the standard optical transceiver module and are not configured or intended to facilitate implementation of, nor access to, additional functionality.
In view of the foregoing, and other, problems in the art, what is needed is an optical module having a flexible and adaptable system architecture that enables ready implementation of functional enhancements to the optical transceiver. Additionally, embodiments of the optical module should also employ a simple but effective communication mechanism so that information concerning processes performed by or in connection with the optical module can be readily and effectively communicated to a host. As well, implementations of the optical module should maintain conformance with established form factors and other standards.